The present invention relates to communications networks. More specifically, it relates to a method and system for estimating and compensating for a flat delay portion of an echo channel characteristic in an echo canceller that uses an adaptive filter to create a local replica of an echo signal.
The central offices of Public Switched Telephone Networks employ network hybrids to interface analog subscribers to the local exchange. Typically, a 2-wire full-duplex line connects the subscriber to the central office while the network hybrid provides an interface to the two 2-wire simplex digital transmission network. In this way, data are transmitted to and received from other subscribers who are connected via a 2-wire line to the digital transmission network by another hybrid at the local exchange.
As is known in the art, echo cancellation is often discussed with reference to speech signal communication between a xe2x80x9cnear endxe2x80x9d and a xe2x80x9cfar endxe2x80x9d and vice versa. A person speaking at the xe2x80x9cfar endxe2x80x9d of a telephone connection has her speech sent over the digital transmission network to a person listening (and eventually speaking) at the xe2x80x9cnear end.xe2x80x9d A portion of the speech signal that is transmitted from the far end speaker xe2x80x9cechoesxe2x80x9d off of the network hybrid that is farther away from the far end speaker and closer to the person located at the near end (the near end speaker). In this way, the xe2x80x9cnear endxe2x80x9d hybrid creates an unwanted signal echo of the transmitted far end speech signal and sends it back toward the far end, only to be heard by the far end speaker as an annoying echo of her own voice.
As is known in the art, network echo canceller circuitry is located at both network hybrids (one at the xe2x80x9cfar end,xe2x80x9d and one at the xe2x80x9cnear endxe2x80x9d). In the case of far end speaker echo, that is, the echo of a transmitted far end speech signal back to the far end, a network echo canceller at the far end network hybrid (the hybrid closest to the far end speaker) is used to cancel the echo. The echo canceller at the far end generates a local replica of the echo generated by the far end signal as it passes through an echo path, or echo channel. This local replica of the echo is generated and used by the echo canceller to attempt to cancel the echo before it returns to the far end speaker. The echo path or channel is the entire path traveled by the transmitted far end signal as it leaves the far end hybrid, as a portion of it echoes off of the near end hybrid, and as its echo returns back to the far end hybrid. In particular, the echo path or channel represents the outgoing and incoming digital transmission lines as well as the near end network hybrid (the one closest to the near end speaker, and distant from the far end speaker).
In the case of echo from the near end speaker, that is, the echo of a transmitted near end speech signal back to the near end, a network echo canceller at the near end network hybrid (the hybrid closest to the near end speaker) cancels the echo that is generated from a portion of the near end speech signal echoing off of the opposite hybrid (the far end hybrid) and returning toward the network hybrid closest to the near end (the near end hybrid).
A typical network echo canceller employs an adaptive digital transversal filter to model the impulse response of the unknown echo channel so that the echo signal can be cancelled. The echo impulse response coefficients used in the transversal filter are updated to track the characteristics of the unknown echo channel. For reference purposes, network echo cancellers are examined from the point of view of the far end speaker, that is, these cancellers act to minimize far end echo.
Connections over the digital transmission network vary in terms of the distance traveled by the signals from near end to far end or vice versa. For example, relative to a far end speaker initiating a telephone call over the digital transmission network, the distant hybrid (at the near end) may be located anywhere from thirty-five or forty miles to thousands of miles away from the closer hybrid (at the far end). The speech signal sent by the far end speaker will encounter a xe2x80x9cround tripxe2x80x9d or xe2x80x9cflatxe2x80x9d delay in its path from the closer hybrid to the distant hybrid and back again as echo to the closer hybrid. Thus, the magnitude and duration of the flat or round trip delay is dependent on the distance traveled by the signals on a particular telephone call, with each telephone connection experiencing a different flat delay. An additional issue is that the magnitude or duration of the flat delay may change during a single telephone call.
The echo canceller at the closer hybrid attempts to cancel the far end speaker echo. The adaptive digital filter used by the echo canceller models the echo channel characteristic including this round trip delay, or xe2x80x9cflatxe2x80x9d delay, portion of the entire echo path delay. Echo impulse response coefficients must be generated to provide a local replica of the far end speaker echo, but since this channel includes the flat delay, many of these coefficients will be effectively zero. Implementing these coefficients in an adaptive filtering scheme is computationally inefficient since a varying proportion of the echo canceller length is used to account for the flat or round trip delay and contributes little or nothing to an individual actual echo replica sample.
Needed is a computationally efficient technique of estimating and compensating for the flat delay portion of the echo path delay in an echo canceller employing adaptive digital filtering to cancel far end speaker echo.
In accordance with preferred embodiments of the present invention, the problems associated with estimating and compensating for a flat delay portion of the echo path delay in an echo canceller, are addressed.
A method for estimating and compensating for a flat delay portion of an echo channel characteristic in an echo canceller is provided. A filter coefficient vector of an adaptive filter that models the echo channel characteristic is updated. An instant flat delay is estimated by analyzing the filter coefficient vector after each update, and periodic averages of the instant flat delay estimate are performed. A final flat delay estimate average is determined when the flat delay estimate average has converged. The length of the adaptive filter is adjusted in response to the final flat delay estimate average.
A second method for estimating and compensating for a flat delay portion of an echo channel characteristic in an echo canceller is provided. A far end signal is received into a buffer. A filter coefficient vector h of an adaptive filter of initial length N that models the echo channel characteristic is updated. An instant flat delay is estimated by analyzing the filter coefficient vector h after each update and determining an index value of the filter coefficient having the largest magnitude. Periodic averages of the instant flat delay estimate are performed. A final flat delay estimate average is determined when the flat delay estimate average has converged and has satisfied an energy requirement. The length of the adaptive filter is shortened in response to the final flat delay estimate average. The far end signal is adaptively filtered using the shortened adaptive filter to create a local replica of a far end signal echo to cancel the far end signal echo.
An improved adaptive filter apparatus that estimates and compensates for a flat delay portion of an echo channel characteristic in an echo canceller is provided. The improved adaptive filter includes a series of delay elements, a filter summer, a coefficient vector generator, a position detector, a periodic averager, and a level detector. The series of delay elements receives a far end signal and the filter summer has initial length N. The coefficient vector generator updates a filter coefficient vector. The coefficient vector generator also shortens the filter summer and the filter coefficient vector and provides a bulk delay element to replace a portion of the series of delay elements, responsively to a final flat delay estimate. The position detector provides an instant flat delay estimate corresponding to the location of the filter coefficient having the largest magnitude after each update of the filter coefficient vector. The periodic averager averages the instant flat delay estimate, and a level detector, responsive to the periodic averager, checks the instant flat delay estimate average to determine the final flat delay estimate.
An improved echo canceller apparatus that compensates for a flat delay portion of an echo channel characteristic is provided. The improved echo canceller includes an adaptive filter, a position detector, a periodic averager, and a level detector. The adaptive filter has an initial length N. The position detector is connected to the adaptive filter and locates the position of the maximum valued filter tap. The periodic averager is connected to the position detector and provides an average of the location position. The level detector responsive to the periodic averager provides a flat delay estimate, D. The adaptive filter shortens from the initial length N to a length Nxe2x88x92D, responsively to the flat delay estimate.
The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.